The invention relates to a connection of a bone screw to a bone plate, the bone screw having a head which lies with a ring-shaped outer surface in contact on a counter-surface of the bone plate and can be fixed with a securing screw which can be screwed into the bone plate in the direction towards the counter-surface, with the bone plate having a passage opening for a shaft of the bone screw.
A problem with these applications consists in that a bone screw which has once been set should form with its head a connection to the implant which is resistant to bending and which is independent of the anchoring forces between the bone screw and the bone. Thus EP-A-0 988 833 shows a connection which is resistant to bending between a bone screw with a spherical head and a bone plate. A further problem for a connection of this kind consists in that it requires a constructional height which cannot be readily reduced. In fields of application such as for example in cervical vertebrae, thin bone plates which are only insubstantially thicker in the region of the connection are advantageous.
It is an object of the invention to improve this situation. This object is satisfied in that the height of the bone plate is less than or equal to the diameter of the shaft in the region of the bone screw; in that the screw head dips into a cut-out of the securing screw; and in that the securing screw terminates with the upper side of the bone plate.
A design of the connection of this kind, which permits constructional heights which correspond at most to the shaft diameter of the bone screw, and which nevertheless results in a rigid connection which is resistant to bending, causes neither unnecessarily thick bone plates nor additionally projecting edges or surfaces which produce pressure points between the implant and the epidermis for the wearer of the implant.
The construction permits providing significantly more than half of the constructional height for the threaded length of the securing screw in spite of the low constructional height of the connection. With a design of the head of the bone screw as a spherical pan and suitable projections and recesses at the securing screw the spherical pan can be pivoted in cut-outs which lie behind the thread of the securing screw. Suitable cut-outs are for example possible when the thread diameter of the securing screw is twice as large as the shaft diameter of the bone screw. This encounters a non-pivotal bone screw which can be anchored in the direction of the axis of the securing screw as well as a pivotal bone screw which is designed with a spherical pan. If the latter with its spherical pan has outer surfaces and inner surfaces with the same sphere center, pivotings from a middle position by angles xcex11, xcex12 can be carried out which can amount to up to about 20xc2x0 in order then to fix the bone screw with a securing screw. This kind of design has the advantage that the fixing of the bone plate can take place largely independently of the angular position of the bone screw. In this the bone plate is not restricted only to the connection of fragments of bone fractures, but can also be a support construction between two vertebrae.
In a use of a bone plate between two vertebrae the bridging over part can be rigid or also formed as a bending spring in order to transmit forces from one vertebra to the other vertebra.
There is also the possibility of designing at two vertebrae in each case a bone plate as anchoring body in the form of a yoke with a plurality of bone screws and to use an elastic band with an elastic pressure body as the actual support construction, as is shown in EP-B-0 669 109.
Bone screws with a shaft diameter of from 2 to 10 mm are suitable as bone screws for the anchoring.
In the following the invention will be described with reference to exemplary embodiments.